Review ArticleScanning Electron Microscopy and Atomic Force Microscopy:Topographic and Dynamical Surface Studies of Blends,Composites, and Hybrid Functional Materials forSustainable Future

Joanna Rydz ,1 Alena Siskova,2 and Anita Andicsova Eckstein3

1Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 41-800 Zabrze, Poland2Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, 845 13 Bratislava, Slovakia3Polymer Institute, Slovak Academy of Sciences, 845 41 Bratislava, Slovakia

Correspondence should be addressed to Joanna Rydz; jrydz@cmpw-pan.edu.pl

Received 2 May 2019; Revised 6 June 2019; Accepted 11 June 2019; Published 10 July 2019

Academic Editor: Candido Fabrizio Pirri

Copyright © 2019 Joanna Rydz et al. *is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Microscopic techniques are often used in material science, enabling the assessment of the morphology, composition, physicalproperties, and dynamic behaviour of materials. *e review focuses on the topographic and dynamical surface studies of (bio)degradable polymers, in particular aliphatic polyesters, the most promising ones.*e (bio)degradation process promotes physicaland chemical changes in material properties that can be characterised by microscopic techniques. *ese changes occurring bothunder controlled conditions as well as in the processing stage or during use indicate morphological and structural transformationsresulting from the deterioration of the material and have a significant impact on the characteristic of materials used in manyapplications, for example, for use as packaging.

1. Introduction

Knowledge of the relationships between structure, prop-erties, function, and performance is essential for pro-spective safe applications of (bio)degradable and/or bio-based polymers in the areas of human health and theenvironment. *e study of the physical and technical basisof the latest developments in the above areas is based quiteoften on microscopic techniques that are also used in avarious industrial applications, including topographic anddynamical surface studies of many materials includingpolymers. Fundamental research and applied research inemerging areas of applications in nanotechnology, in-terfacial science and engineering, advanced production,catalysis, bioengineering, bioinspired synthesis, greenproduction routes, sensing, and actuation are often alsobased on microscopic techniques. Particular emphasisshould be placed on environmentally friendly blends,composites, and hybrid materials from renewable resources

without adverse environmental impact, with a short globalcarbon lifecycle, with green production routes and/orsuitable to recycle, materials based on natural, renewable,and synthetic polymers for a sustainable future [1–3].

In recent years, the focus has been taken on the de-velopment of novel atomic force microscope (AFM) andscanning electron microscope (SEM) based methods. *eSEM and AFM are powerful characterisation tools inpolymer science, capable of revealing surface structures.SEM provides a three-dimensional (3D) image with highresolution and is used to characterise the morphology of thesample surface, particle size, microorganism, and fragments.Energy-dispersive (EDS) detector can additionally providesemiqualitative and semiquantitative information on ele-mental analysis of the surface (topochemical data) andidentification of additives and impurities to detect con-tamination (residues) as well as determine the origin ofsample damage [4]. SEM is a powerful visualisation tool usedin material science, including in the field of polymer

HindawiAdvances in Materials Science and EngineeringVolume 2019, Article ID 6871785, 16 pageshttps://doi.org/10.1155/2019/6871785

sciences. Due to the high lateral resolution, large depth offocus, and the facility for X-ray microanalysis, it provides aconsistent image of the polymer blends and compositesmorphology as a nonuniform structure characterised bypolymer variable thickness and variable density. For SEManalysis, the surface of nonconductive samples must becoated with a thin layer of gold or platinum. Sometimes,surface pretreatment is carried out by ion sputtering orchemical etching to reveal structural details. In addition,brittle fracture (in liquid nitrogen) can provide informationabout the internal morphology of polymer matrix. SEMmicrographs show that polymer blends and composites canhave different surface features and heterogeneous localdensity of chemical components. SEM images also showsurface defects such as cracks, etching residues, differentialswelling, depressions, and perforations [5]. However, SEMdrawback is that during imaging, the electron beam maypermanently damage the observed sample. Degradation, anundesirable effect, can alter or destroy details and conse-quently change the results and conclusions [6]. By means ofAFM technique, in addition to topography imaging at thenanometre scale, the phase imaging mode is actively in-volved in the mapping of surface heterogeneity of theblends, composites, and hybrid materials because the phaseresponse of the cantilever is sensitive to the surfaceproperties, such as adhesiveness, friction, electrical forces,capacitance, magnetic forces, conductivity, viscoelasticity,surface potential, and resistance without need for samplepreparation or vacuum environment [7, 8]. It has beenshown that AFM is able to provide both qualitative andsemiquantitative information on the deterioration andageing of materials during degradation [9]. In phase modeimaging, different samples exhibit different interactionswith the AFM tip, so the phase shift changes.*e propertiesof the sample affected by interactions with the AFM tipinclude friction, adhesion, and high elasticity. Using phasemode, images show differences between the local regionson the sample, and this mood is also useful for testing theconsistency of coatings and for displaying cracks and otherdegradation features [10].

Analysis of polymer blends and composites morphologyand surface changes (erosion) using AFM and SEM tech-niques gives a lot of information about various processes andphenomena. *e use of a microscope to monitor a numberof aspects of the surface degradation of polymer blends andcomposites in various selected degradation environmentsallows to track the relationship between surface topographyand degradation patterns of materials and provide reliableparameters for predicting of long-term degradation. Like-wise, miscibility of blends is one of the most importantfactors affecting the properties of polymer materials. *esurface structure and morphology of (bio)degradablepolymer blends have a great impact on the degradationprocess. Moreover, microscopic observation is a powerfultool to detect 3D printing defects and downstream pro-cessing problems, which can also affect the course of deg-radation of printed items [11]. Farther imaging techniques,such as SEM and AFM, have been developed to observedmaterials of submicron size, so they can be used to determine

microscopic characteristics of nanomaterials such as shape,size, surface morphology, crystal structure, and dispersionand are an important part of determining phase purity,electronic transition plasmonic character, atomic environ-ment and surface charge, etc. [12, 13]. Also, microscopicexamination of electrospun (bio)degradable polymers allowsunderstanding the relationships between processing pa-rameters, morphology of the samples, and their properties.*e formed mats have unique characteristics such as highporosity, surface area to volume ratio, permeability, po-rosity, stability, ease of functionalisation, and excellentmechanical performance [14, 15].

*e development of (bio)degradable polymeric materialsfor many new advanced applications is of great importancenot only for usable reasons, but also from the point ofenvironmental protection and sustainable development. *eefficient use of these polymers requires basic researchnecessary to determine the relationship between the struc-ture of such materials, properties, and mechanism of deg-radation. *erefore, the following overview presents themost important issues in the testing of the behaviour of (bio)degradable polymeric materials in various environmentsusing surface imaging techniques, which have not beenpresented in one review so far. Microscopic techniques are atool for studying changes in the surfaces of materials underinvestigation during various processes and assessingmicrodamage, such as crack propagation, grain size change,and capability, which allows to provide qualitative andsemiquantitative information on degradation and ageing.*ey are also a sensitive tool to detect the deterioration of thematerials tested and characterise mechanisms of the blendsand composites reinforcement.

2. Microscopic Investigation of Eco-FriendlyPolymer Blends and Composites

2.1. Ageing and Degradation Processes. AFM allows for amicrodestructive quantitative distinction between the initialand the final stage of degradation [4]. *e polymers beforedegradation generally have a fairly flat and smooth surface.During degradation, the surface becomes rougher. *edissolution of degradation products and erosion usuallycreate cracks and pores which cause an increase of rough-ness. Small cracks, formed by water absorption (in case ofhydrolytic degradation in aqueous media) or simply erosionof the polymer, increase as contact with water leads tohydrolysis, and locally produced acids catalyse degradationand cause polymer dissolution inside the pores. *e depthand/or the number of pores in the surface increased withtime, conditions, and degradation media. *e surface ero-sion depends also on the solubility of the low-molar-massdegradation products [16].

Under natural conditions (compost and weather con-ditions) at the beginning of degradation, cracks on thesurface of the polymeric material appear, after which thematerial begins to disintegrate into pieces in accordancewith cracks (disintegration). Degradation under naturalconditions of the polylactide (PLA) and PLA/PHB (PHB:poly(3-hydroxybutyrate)) blend, as well as their composite

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materials proceeds according to a different mechanism,which reflects the microscopic images. *e cracks on thePLAmaterial surface appear, and then the erosion occurredinside deeper part of the material, which indicates a water-catalysed hydrolysis mechanism. For a PLA/PHB blend, thehomogeneously licking of the whole surface, layer by layer,is observed, which indicates an enzyme-catalysed hydro-lysis mechanism [8, 17, 18]. *e degradation of polyestersin a biological environment, including anaerobic andaerobic conditions, activated sludge, public wastefields, andcomposting facilities, results from enzymatic attack (in thepresence of specific enzymes produced by microorganismssuch as bacteria and fungi) or simple hydrolysis (water-catalysed cleavage of ester bonds) or both. In contrast tobacterial polyesters, such as polyhydroxyalkanoates(PHAs), which directly undergo biodegradation by manymicroorganisms present in the environment, the high-molar-mass PLA is colonised by relatively few microor-ganisms and more frequently takes place the water-cata-lysed hydrolysis of this material. But in general, for (bio)degradable polymers, the first step of biodegradation undernatural and laboratory conditions is a chemical hydrolysis(mainly through random chain scission), which leads to areduction of molar mass. In the next step, called miner-alisation, the oligomers formed are bioassimilated by mi-croorganisms as an energy source [19].

Figure 1 presents SEM micrographs of the surface of thePHA films treated with (i) lipase solution (0.1 g/L) under 30°Cand pH� 7.0 for 24 h and (ii) 1N NaOH at 60°C for 1 h [20].

Many microorganisms, such as extracellular PHBdepolymerase, excrete enzymes outside their cell walls. *eenzymes first adsorb onto the surface of polymer materialsthrough their substrate-binding domains at the C-terminusand then catalyse the hydrolysis of polymer chains by theircatalytic domains at the N-terminus [21]. *e enzymes areable to cleave specific bounds in the polymer chain on thesurface of the polymer material, which causes the de-struction of this material layer by layer (surface erosion, doesnot occur inside the polymer matrix) (see Figure 1, lipase)[22]. Nonenzymatic hydrolysis of PHAs proceeds from thesurface towards the deeper layers. *e surface becamerougher, and the density of the holes forming on the surfaceincreased (see Figure 1, NaOH) [21].

Investigation of biodegradation of PHA/wood com-posites using SEM in secondary electron mode showed thatdespite the low-molar-mass loss, which is characteristic inthe first stage of biodegradation, when the surface degrades,layer by layer, the surface of plain PHA had a clearly roughsurface with dents after 12months of burial in soil. *esurface of PHA composite with 20% wood was also sig-nificantly rougher after soil burial. A network of fungalhyphae filaments has been embedded in the matrix with thepresence of spores. *e decreases in molar mass of PHAthroughout 12months of soil burial became more severe asthe wood content increased [23].

*e degradation of PLA and its composites with starch incompost and soil also indicates the process catalysed byenzymes in the case of composites. SEM micrographs (seeFigure 2) showed changes in surface morphology (cracks) of

PLA and its composites after degradation in compost andsoil for 14 days. (Bio)degradation occurred for all polymersamples tested, but more changes were observed in PLA/starch (50/50) composite than on pure PLA samples. *ismeans that the addition of starch to PLA resulted in theirgreater biodegradability. Starch was eroded from the surfaceof the PLA blends, while for pure PLA, degradation occurredin the deeper part of the polymer, suggesting a preferentialmechanism of chemical hydrolysis. Degradation occurredfaster in compost than in soil [24].

*e EDS detector can be used to identify of additivesaffecting the course of degradation. It was found that dif-ferences in the rate of degradation of PLA/PBAT (PBAT:poly(butylene adipate-co-terephthalate)) blends with asimilar content of the PLA component may be due to dif-ferences in their thickness and/or the presence of com-mercial additives used during processing. *e resultsindicated that the presence of talc may interfere with thebehaviour of materials towards water and consequently altertheir degradation profile. EDS analysis of selected regions ofPLA/PBAT blends showed the presence of white inclusions(Mg and Si) in the polymeric matrix derived from a com-mercial additive, hydrated magnesium silicate (talc) (seeFigure 3) [25].

2.2. Effects of Miscibility. Observation of the surface mor-phology by AFM can confirm different miscibilities of thepolymer material [8, 26]. *e blends with poor miscibilityexhibit phase separation of the components, while a ho-mogenous surface morphology is observed for the blendwith good miscibility (see Figure 4) [16]. Miscibility andcompatibility of polyester blends containing PLA and PHAs,such as PHB or poly(3-hydroxybutyrate-co-3-hydrox-yvalerate) (PHBV) depend on the molar mass and crystal-linity of the components, on the blending method and thecomposition of the blend. *e blends are immiscible whenthe molar mass of components is high [27]. A decrease ofmolar mass of one of the blend components results in areduction of the phase separation [28].

*e diameter and depth of the pit domain of immiscibleblends increased as the poly([R,S]-3-hydroxybutyrate)((R,S)-PHB) component increased in the blends, as observedby AFM. Only in the blend with good miscibility, the surfacewas completely smooth. *e degree of erosion of thepolymeric material is strongly dependent on the blendmiscibility of the two components. *e poor miscibility ofthe blend with the largest pit domain before degradationpromoted greater roughness of the surface during degra-dation [16].

*e cocontinuous phase structure of polymer blend canbe confirmed by means of SEM. SEM was used to analysethe effect of a preparation method on the miscibility ofPBAT and poly(hydroxy ether) (PH) of bisphenol A blendsand the effect of a nondegradable component (PH) in thedegradation of PBAT. A continuous morphology of the50PBAT/50PH blend prepared by casting can be observed.*is type of morphology was attributed to phase inversioncomposition. *e morphology of 25PBAT/75PH has a

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Untreated

PHB

PHBHHx

Lipase NaOH

Figure 1: SEMmicrographs of PHB and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) films before degradation (untreated)and after lipase and 1N NaOH catalysed hydrolysis (adapted with permission from ref. [20]).

Before degradation

PLA

PLA/starch(50:50)

A�er degradationIn compost In soil

(a) (c) (e)

(d) (f)(b)

Figure 2: SEM images of PLA and PLA/starch (50/50) composite samples surface before degradation (a and b), after degradation in compost(c and d), and after degradation in soil (e and f) for 14 days (adapted with permission from ref. [24]).

Figure 3: SEM-EDS micrograph of PLA/PBAT (40/60) blend surface at a magnification of 2500x (adapted with permission from ref. [25]).

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biphasic structure: the adhesion between the two com-ponents is very good; however, the blend is immiscible. Asexpected, the rate of degradation decreased with the ad-dition of PH [29].

*e effect of melt viscosity of the two melt blendedpolymers, PLA and PBAT, on the morphology and me-chanical properties of obtained (bio)degradable blends wasinvestigated by means of SEM. Based on the relative meltviscosities of the PLA and PBAT in the processing method, itis possible to calculate the volume fraction in which acocontinuous phase structure is formed. *e SEM image ofthe fracture surface from the tensile tests for pure PLAexhibited a flat, featureless structure that is typical of brittlefracture. In PBAT/PLA blends with PBAT between 20 and40wt%, a cocontinuous phase structure was observed; thefibrils were drawn from the fracture surfaces, which is acommon feature of ductile failure. Since PBAT has a muchlower yield stress than PLA and undergo plastic deformationat lower stress, these fibrils were caused by a continuousPBAT phase. When PBAT content in the PBAT/PLA blendreached 60wt%, the continuous PLA phase was no longervisible. Large PLA particles were dispersed in a continuousPBAT phase, and they deboned from the matrix, whichcaused cracks and flaws at the interface, resulting in rela-tively poor mechanical properties. When the PBAT contentreached 80wt%, the blend exhibited very ductile behaviourin the tensile test. PLA was still dispersed in the PBAT in theform of particles; however, the particle size was muchsmaller and the dispersion became much more uniform.*eimpact of PLA on the mechanical properties was un-important, and the blend behaved in a similar way to purePBAT (see Figure 5) [30].

2.3. -ree-Dimensional Printing: Defect Detection. 3Dprinting or additive manufacturing (AM) refers to processesused to create objects in 3D using digital data of the 3Dmodel. To form 3D objects with almost any shape or ge-ometry, subsequent layers of material are applied [31]. 3Dprinting is a valuable alternative to traditional processingmethods in the creation of various products, such as scaf-folds for regenerative medicine, artificial tissues and organs,electronics, components for the transportation industry, artobjects, etc. *is technique has demonstrated exceptionalcapacities for producing complex structures with preciselytailored physical and mechanical properties, biologicalfunctionality, and an easily customisable architecture [32].

Subsequent layers may, however, contain structuralinterruptions or defects that negatively affect the reliabilityof the 3D printed object [31]. Also, build direction has asignificant effect on the structure andmorphology and henceon the properties of elements made using 3D printing (seeFigure 6) [33].

When samples are printed horizontally, upper andunderside layers have surfaces with a different characteristic.*is is due to the fact that one layer is in contact with the 3Dprinter platform. In the case of fused deposition modelling,rapid prototyping technique, the heated thermoplasticpolymer filament is extruded from a tip that moves in the XYplane. *e controlled extrusion head deposits the polymermaterial onto the printer platform, first forming the un-derside layer. *e printer platform is kept at not too highconstant temperature (around 65°C for polyesters), wherebythe thermoplastic material hardens quickly. However,polymeric material is permanently maintained at a constanttemperature during printing which can affect properties of

150.0nm

75.0nm

0.0nm

(a)

150.0nm

75.0nm

0.0nm

(b)

Figure 4: Selected 6× 6 μm AFM images of the surface erosion of PLA, 97PLA/3(R,S)-PHB, 91PLA/9(R,S)-PHB, and 85PLA/15(R,S)-PHBblend samples, respectively, before (a) and after (b) 52weeks of the degradation process in paraffin (adapted with permission from ref. [16]).

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0/100

(a)

20/80

(b)

40/60

(c)

60/40

(d)

80/20

(e)

80/20

(f )

Figure 5: SEM micrographs of fracture surfaces from the tensile tests of PBAT/PLA blends (adapted with permission from ref. [30]).

(a) (b) (c)

Figure 6: SEM micrographs of upper (UH) and underside (BH) layers of samples surface obtained by 3D printing in horizontal (H) andvertical (V) directions (adapted with permission from ref. [33]).

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the final element from (bio)degradable polymers, such asPLA or PLA/PHA blend. *e extrusion head deposits thematerial layer by layer in the Z-axis.*e process is continuedto produce the desired element [34–36]. *e upper layer isthe furthest from the platform (see Figure 6, UH layer).Vertical direction does not affect the structure of the surfacesof the PLA or PLA/PHA blend because the contact with theprinter platform was only at the small area at the base of thesample.

Tissue engineered scaffolds must have an appropriatelyorganised and repeatable microstructure that allows thecells to be assembled in an ordered matrix that ensures theproper perfusion of nutrients. *is is what 3D printingtechnology provides. 3D cylindrical scaffolds made ofpoly(ε-caprolactone) (PCL) were prepared using a bio-plotter dispensing machine. In order to obtain a porous butsufficiently stiff 3D structure, the scaffold consisted offilament layers, which were extruded sequentially by thebioplotter with 0°/90° pattern. *e diameter of the filament(200 μm) depended not only on the internal diameter of thenozzle (250 μm) but also on the deposition speed (30–35mm/min) (see Figure 7) [37].

SEM images allow not only to characterise the mor-phology of the 3D samples surface but also filament size ormicroorganism species.

3. Microscopic Methods inNanomaterial Characterisation

Nanotechnology is one of the fastest growing in-terdisciplinary areas at present. Nanomaterials are mainlyused in electronics, healthcare, cosmetics, construction, andautomotive industries. Depending on the application,nanoparticles are divided into two main groups: organic andinorganic. Nanoparticles with inorganic base are fullerene,quantum dot, silica, and gold and with organic base aremicelle, dendrimer, liposome, hybrid, nanosphere, andnanocapsule [39].

(Bio)degradable polymers are often used in the prepa-ration of nanomaterials [40]. *e applications of (bio)de-gradable polymeric nanomaterials have a wide range ofusability in the field of therapeutics such as diagnostics,imaging, drug delivery, organ implant, tissue engineering,and in area of packaging materials [41]. *e type of polymersystem, area of applicability, and required particle size de-termine the type of (bio)degradable polymeric nano-materials preparation.

*e SEM/EDX technique is useful in the research in allworks that require the determination of elements, endog-enous or exogenous, in tissues, cells, etc., such as drugsdelivery. *e EDX helps in the detection of nanoparticlesused to improve the therapeutic efficacy of certain che-motherapeutic agents. It is also used in the study of envi-ronmental pollution and in the characterisation of mineralsbioaccumulated in the tissues [42]. Using SEM can be ex-amined, for example, the fine nanoporous aerogels structure,in situ SEM methods investigating the thermal stability ofnanoparticles such as graphene/Cu based materials, theeffects of electron beam irradiation on the electrical

properties of carbon nanotube yarns, and the nano-indentation work of multiphase thermoelectric material[43]. AFM is used to investigate the size and shape ofnanoparticles in 3D mode, to assess the degree of surfacecoverage with nanoparticles, dispersion of nanoparticles incells and other matrices/carriers, and precision in lateraldimensions of nanoparticles [44].

*e most commonly applicable (bio)degradable poly-mers are PLA, poly(lactic-co-glicolic acid) (PLGA), andPHBV which themselves form particles or are used as part ofcopolymers or surface materials for inorganic particles.Biocompatible PLA-based micro- and nanoparticles areprepared by different techniques (Table 1), and the mor-phology of these nanoparticles is most often investigated bySEM.

PLGA is another widely used (bio)degradable polymerfor nanoparticle preparation by the emulsification, solventevaporation, and nanoprecipitation method [53]. By usingvarious molar masses of PLGA (Mw of 14,500, 45,000,85,000, 137,000, and 213,000 g/mol), particles were preparedwith size 90–120 nm. *e prepared particles were studied asa drug release system [54].

*e effect of various preparation conditions on the sizeand morphology of the PHBVmicro- and nanoparticles forpotential applications as reinforcement of PHBV/starchmatrices was examined using field emission SEM (seeFigure 8). *e spherical porous micro- and nanoparticlesproduced by the emulsification/solvent evaporation werewith a size of 300 to 20 μm. It has been found that the size,porosity, and the particle size distribution can be con-trolled by the choice of surfactant and polymer concen-tration during the emulsification process, while choosingthe appropriate antisolvent and adjusting its polarity werecrucial for obtaining spherical particles through nano-precipitation [55].

Studying the cell material interactions is crucial forcollecting relevant information on the impact of structureand composition at the atomic and macromolecular levelof bioinspired scaffolds. *us, the assessment of thesurface properties can be suitable technique in thisregard. AFM has a significant impact on the in vitrostudies of (bio)degradable materials produced in variousforms, i.e., films, fibres, or nanoparticles for tissue en-gineering and drug delivery, capable of gradual re-sorption by ex novo formation of extracellular matrixwith predefined structural/mechanical properties, similarto native tissues [56].

4. Electrospun Eco-Friendly Polymer-BasedBlends, Composites, and Hybrid Materials:Microscopic Examinations

Electrospinning is considered as attractive procedure offorming ultra-fine fibres in a nano- and microscale intononwoven mats due to its simplicity, cost effectiveness, andhigh rate of production. *e schematic representation of themost common electrospinning setup is displayed in Figure 9.*e advantage of electrospun nanofibres is mimicking of

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three-dimensional structure of natural extracellular matrix,which predetermine them for biomedical and biotechnologicalapplication as tissue engineering, wound dressing, and drugrelease [57, 58]. Besides this, it can be synthesised and tailoredto suit a wide range of others applications including electronics[59, 60], environmental engineering [61, 62], agriculture[63, 64], and food packaging [65, 66].

*e high customisation and easy functionalisation of thenanofibres offer numerous opportunities to control and toevaluate the morphology and/or chemical changes on thesurface of blends, composites, and hybrids. SEM, trans-mission electron microscopy (TEM), and AFMmay be validtools for characterising surface topography and un-derstanding the specific mechanical, chemical, and physical

(a) (b) (c) (d)

(e) (f) (g)

Figure 7: SEM micrographs: (a) lateral view of 3D filament-deposited scaffold reconstruction; (b) inner structure of the same scaffold;(c) colonisation of the 3D PCL scaffold in dynamic condition (scale bar 1mm); (d) high magnification showing the external cell monolayer(scale bar 1mm); (e) cells bridging the grooves (scale bar 100 μm); (f ) cell arrangement suggestive of a new vascular structure (scale bar100 μm); (g) spheroid of MC63 and human umbilical vein cells generated in dynamic condition (scale bar 50 μm) (adapted with permissionfrom ref. [38]).

Table 1: Preparation of PLA base (bio)degradable polymeric nanomaterials.

Preparation techniques Polymers Size (nm) ReferencesEmulsion PLA 200 [45]

Nanoprecipitation Polydopamine-modified tocopheryl poly(ethyleneglycol succinate)-PLA (TPGS-PLA) 126 [46]

Dialysis PLA-poly(ethylene glycol)-PLA (PLA-PEG-PLA) 90–330 [47]Spray drying PLA 960–3000 [48]Melting technique PLA/PLGA Micro [49]Supercritical fluids technique PLA Micro [50]Microfluid technique PLA/c-Fe2O3 Micro [51]Template/mould based technique PLA Micro [52]

Figure 8: Micrographs of micro- and nanoparticles of PHBV prepared by nanoprecipitation method using 0.5% PHBV in dime-thylformamide (DMF) as solvent and water as antisolvent (a) and 0.1% PHBV in DMF as solvent and 10% NaCl solution in water asantisolvent (b); microparticles of PHBV prepared by emulsification/solvent evaporation method using 1% PHBV in dichloromethane(DCM) and sodium dodecyl sulphate (SDS) as surfactant in a concentration of 1% (c) (adapted with permission from ref. [55]).

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properties with regards to application of investigated ma-terials [56, 67, 68].

4.1. Electrospinning Process Parameters of (Bio)degradablePolymers. To date, many materials including natural poly-mers, synthetic polymers, and their mixture have beenelectrospun. Particularly, (bio)degradable polymers are usedextensively in the biomaterial fields including polyesters,such as PLA, poly(glycolic acid) (PGA), PCL, PHB, orpolyester copolymers, such as PLGA, PHBV, and PBAT[63, 69, 70].

*e parameters influencing the morphology and prop-erties of electrospun fibres can be divided into solutionparameters, process parameters, and ambient conditions.*e most relevant parameters related to the solutionproperties are nature of used solvent, its dielectric proper-ties, volatility, boiling point, the solution concentration, andmolar masses of polymers that control the viscosity. Into thegroup of processing parameters belong flow rate through theneedle, inner diameter of the needle, needle-to-collectordistance, applied voltage, and geometry of collection (staticcollector or rotating drum). *ese parameters control the jetformation. *e ambient conditions as ambient temperature,humidity, and air flow are influencing the morphology of theproducts as well [57, 71, 72]. After electrospinning, SEM isperformed for morphological analysis generating SEMimages, for measuring the fiber diameter, etc. SEM enablesthe optimisation of electrospinning process for preparationof continuous nanofibres with specificmorphology and well-defined physical and mechanical properties, depending onthe type of application of the mats.

Most of the studies concern the electrospinning pa-rameters for (bio)degradable polymers trying to understandthe electrospinning mechanism. PLA, PCL, and PHB andtheir blends in the form of nanofibres can be used in a widevariety of biomedical and biotechnological applications.PLA as well as PCL and PHB electrospinning processstrongly depends from the type of polymer/solvent system.*ese (bio)degradable polymers are electrospun from thedifferent single solvent system such as acetone, 2,2,2-tri-fluoroethanol (TFE), chloroform (TCM), and many others,and continuous fibres are obtained only using solvents withhigh electrical conductivity. Smooth defect-free nanofibreswith a narrow and more homogenous diameter distributionare collected using binary solvent systems such as acetone/DMF or acetone/dimethylacetamide (DMAc) (in the case ofPLA) [72], tetrahydrofuran (THF)/methanol, DCM/DMF

(in the case of PCL) [73, 74] (see Figure 10), and TCM/DMF(for electrospinning of PHB) [71].

In case of PCL electrospinning, the variable parameterssuch as single solvent system, DCM, binary solvent systemDCM/DMF 1/1, and concentrations 5wt.%, 10wt.%, and15wt.% of polymer solution were investigated, respectively.

(Bio)degradable copolymer PLGA was successfullyelectrospun from DCM and TCM but from hexa-fluoroisopropanol (HFIP) was generated the narrower fibrediameter distribution [75]. PHBV nanofibrous mats weregenerated from TCM [76]. Blend of two (bio)degradablepolymers PLA and PBAT was electrospun, and the bestsolution for achieving the smooth fibres was the binarysolvent DCM/DMF, but the proportion played an importantrole, and 3 : 2 was found as the most suitable [77]. Solutionconcentration which is closely related to viscosity also playsan important role in electrospinning. *e increasing con-centration leads to the increase of fibre diameters. Too smallconcentration prevents the nanofibres formation, and in-stead of fibres, only beads appear [78]. With regards toprocess parameters of electrospinning of (bio)degradablepolymers, in general, at the higher voltage, the jet is unstable;therefore, more homogenous nanofibres at lower appliedvoltage are formed. In the case of PCL solution in HFIP, thesmooth fibres are generated at 15 kV compared to 25 kVwhen the fibres with high average diameter are formed [79].Tip to collector distance has a direct effect on the jet flighttime and electric field strength. Fibres with small diameterscan be formed when the working distance is greater. Re-ducing the distance shortens the time of jet flight, time ofsolvent evaporation, and electric field strength; all theseconditions result in an increase of bead formation. On thecontrary, a very large increase in the needle tip distance tothe collector leads to an unstable jet and to beaded structurednanofibres [76].

*e ambient conditions affect the morphology of thenanofibres observed by SEM. *e collection procedure wasstudied by SEM. *e average diameter of nanofibres col-lected on static collector was compared to the average di-ameter collected by rotating drum. *e difference lies indirecting of the fibres in the mats. From static collector wasobtained mats with random placement of the fibre, whilst byrotating drum the aligned fibres in mats were collected. *eaverage diameter of random (559.04 nm) and aligned PHBnanofibres (675 nm) was investigated, and it was shown thatthere were no significant differences in the diameters. On theother hand, the PLA is very sensitive on humidity. In thespinning process, solvent evaporates from charged jet, but

Piston

Pump

SyringeApplied voltage

Forming fibres

Electrospunfibres

CollectorHigh-voltage supplier

Figure 9: Schematic representation of conventional electrospinning setup to prepare electrospun nanofibres (archive of authors).

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water vapour occupies the position of the evaporated sol-vent. As a result, pores are formed on fibre surface aftersolidification of the polymeric phase (see Figure 11). *ebigger pores formed at higher humidity due to more watervapour replacing the position of the evaporated solvent [80].*e electrospinning mechanism is difficult to understanddue to lot of parameters. Microscopic examination enablesto adjust the conditions to achieve the required morphologywith regards to the application.

Highly porous PLA was electrospun into the liquidcollector, hot water bath, at 70°C.*e 12wt.% solutions wereprepared by dissolving PLA pellets in TCM. *e solutionwas stirred on a magnetic plate with intensity 615 rpm for2.5 h. DMF was added to reach required concentrations andfor enhanced properties of the solutions. *e TCM/DMFratio was set at 90/10 v/v.*e parameters for electrospinningPLA were selected. *e solution was electrospun from a

5mL syringe at the flow rate of 1.2mL/h. Voltage of 12 kVwas applied; needle to collector distance was 15 cm. *eaverage diameter measured from SEM micrographs calcu-lated by ImageJ software was 893± 151 nm.

4.2. Microscopic Investigation of Blends, Composites, andHybrids Based on (Bio)degradable Polymers. Electrospunnanofibres are mimicking of structure of natural extracel-lular matrix, which predetermine them for application inbiomedical field as wound dressing, tissue engineering, anddrug release.

*e fundamental requirements on the biomaterials usedin biomedical field are biocompatibility, mechanical sta-bility, and specific biological properties. Conventional singlepolymer materials cannot meet these requirements; there-fore, multicomponent systems are designed and prepared.

DCM DCM/DMF 1/1

5%

(a) (b)

(c) (d)

(e) (f)

10%

15%

Figure 10: SEM micrographs of electrospun PCL at constant conditions: applied voltage� 10 kV, flow rate� 1ml/h, needle to collectordistance� 15 cm, and scale bar� 5 μm (archive of authors).

10 Advances in Materials Science and Engineering

Introduction of biomolecules or inorganic molecules into(bio)degradable polymer matrix is effective to obtain blends,composites, and hybrids with required properties [81].

4.2.1. Wound Healing Dressing. Porous electrospun nano-fibrous matrices offer greater surface area and allow highoxygen permeation, easy protrusion of exudates, and pro-tection of wounds from infection, bacterial colonisation, anddehydration. All these properties are important for woundhealing dressing. For such application was tested compositecontaining starch and PCL, nanofibres, which was fabricatedby coaxial needle electrospinning technique. Processingparameters such as polymer solution concentration, flowrate, the nozzle-ground distance, or applied voltage had amarked influence on the composite fibre diameter, andmorphological study had an importance for the visualisationand understanding of the fibre structure. *e SEM visual-isation revealed that the PCL component in the electrospunnanocomposite structure was more prone to create fibres,while the starch formed beads.*is was because of structuralfeatures of starch. Furthermore, a relationship betweendiameter of the fibres and concentration of starch was ob-served. *e fibre diameter increased with the starch con-centration because the bigger beads caused the formation ofthicker fibres connecting these beads, also providing betterstrength. A lower diameter range of fibres was caused byboth low viscosity and electrical conductivity, resulting in alow viscoelastic force. In general, a higher concentration ofstarch denoted the higher viscosity, fibre diameter, and beadformation [82]. In other study, the electrospun mats on thebase of blends containing PHBV copolymer and collagen aswell as gelatine, respectively, were examined as a biologicalwound dressing. SEM images showed cell morphology andattachment after in vitro culture of human dermal sheath(DS) cells in matrices. DS cells adhered significantly faster tothe hydrophilic PHBV/collagen matrix than to hydrophobicPHBVmatrix. Whilst DS cells achieved complete confluenceinto the hydrophilic matrix within 6 hours, at the same time,

these cells were just beginning to attach to the hydrophobicmatrix. Nevertheless, wound healing test results showed thatthe contribution of PHBV/collagen into the healing processwas small, although it showed increase in hydrophilicity andfaster hydration, better cell attachment, and proliferationcompared to PHBV, which was much more mechanicallystable. *ese results indicated that mechanical stability ofmatrix is more important factor in wound healing than itscell culture activity [83].

4.2.2. Drug Delivery Systems. Electrospinning offers theopportunity to design new systems which act as a vehicle oflocal drug administration that can release the therapeuticagent at the site of the pathogenic area. Drug delivery on thebase of polymer nanofibres is based on the principle that thedrug dissolution rate increases with increased surface area.*e nanofibrous membrane containing drug can be post-processed into the kind of drug formulation [84].

Biomimetic fibrous scaffolds of PLA and PLA loaded withdipyridamole (DPM) were developed to act as drug deliverysystem coated on the cardiovascular stents [74]. *e evalu-ation of the surface morphology and topography of PLAscaffold after fabrication, DPM loaded PLA system, coatedstent, and delivery system during degradation process wasconducted through AFM and SEM. Drug-loaded scaffoldswere fabricated with good morphology without beads. *eaverage diameter of blank PLA nanofibres (522 nm) andloaded PLA (556 nm) was calculated by ImageJ software onthe basis of SEM images. *e degradation study took 90 days,and how scaffolds degrade with hydrolysis was observed.Microscopic evaluation revealed the differences in averagediameter of nanofibres. After 30 days, the diameters in bothcases (blank and loaded PLA scaffolds) increased due to theswelling of the scaffolds, along the hydrophobicity of thepolymer; then in 90 days, the degradation started due tohydrolysis and therefore the diameter decreased. Finally, thetotal amount of DPM loaded was released through scaffoldafter 218 days [74]. Electrospun mats for controlled release of

0

10

20

Cou

nt

30Average = 893SD = 151

40

1000 1500 2000 2500 3000 3500 4000 4500500Diameter (nm)

(a) (b)

Figure 11: SEM micrographs of electrospun highly porous PLA (a) and histogram (b) (archive of authors).

Advances in Materials Science and Engineering 11

paracetamol were also developed.*e SEM study showed thatthe morphology of electrospun fibres was of a commonnature, and they are round-shaped, bead free, randomlyarrayed, fibrous, and highly porous mats. *e paracetamolcrystals were not detected on the surface of fibres. *is in-dicated that the drug was incorporated into the structure ofelectrospun fibres homogenously. From SEM images, averagediameters were calculated. *en, degradation profiles of fi-brousmats were assessed by SEM. Compared to the fine fibresbefore incubation, the morphology during incubationchanged significantly.*e fibres after 24 h of incubation at pH5.6 were swollen and curly, and the samples containedcoexisting fibres and films. At pH 4 after 24 h of incubation,only films were detected without any fibres evidence [77].Drug delivery vehicles can be used also in food packaging withloaded various bactericide agents and compounds with an-timicrobial and/or antifungal activity [65, 66].

4.2.3. Tissue Engineering. Tissue engineering or regenerativemedicine uses the scaffolds to support the cells to regeneratenew extracellular matrix which have been destroyed by dis-eases, injury, or congenital defects. Electrospinning provides3D porous mats with high porosity and large surface area,mimicking extracellular matrix structure; therefore, they areconsidered an excellent candidate for use in regenerativemedicine.*e requirements for tissue engineering applicationare biocompatibility and (bio)degradability. Also, scaffoldarchitecture affects cell binding significantly [58]. (Bio)de-gradable polymers are commonly used as biomaterials forbone repair. To date, PLA, PLGA, PCL, PGA, their blends,composites, and hybrids were investigated for tissue engi-neering because they meet the most criteria for this appli-cation [85]. *e bone is a rigid and complex form containingfibrous organic matrix impregnated with inorganic minerals,such as calcium or phosphate. Inorganic minerals provide thehardness and toughness to tissue. For example, alternatively,interest has been paid to a hybrid from PCL fibrous scaffold incombination with silicate-containing hydroxyapatite (SiHA)microparticles to improve cell penetration [86]. *e mor-phology and structure of fibres is important in controlling theadhesion and proliferation of cells; therefore, the surface wasinvestigated by SEM and was characterised by presence of thefibres in micron scale for PCL-SiHA scaffolds. *e cell via-bility, as a crucial issue for the clinical use of 3D scaffolds, wastested by human mesenchymal stem cells (hMSCs) for a site-specific repair [87].

4.2.4. Electronics and Bioelectronics Applications.Electrospinning is a popular method also in the field ofsensing or electronics, in general. *e (bio)degradablematerials are preferred in this direction as well because manystudies are focused on creating and investigating environ-mentally friendly conductive sensors for gases and volatileorganic compounds based on (bio)degradable electrospunnanofibres. To design and fabricate of environmentallyfriendly conductive sensors in moistened environments, the(bio)degradable electrospun nanofibrous polymer blendscontaining polyaniline (PANi) and PHB [88], PANi and

PLA [89], PLGA, PCL and PANi [90] or composites con-taining silver ink, PCL and poly(glycerol sebacate) (PGS) forstretchable electronics [91], and electrospun PCL nanofibresmodified by polypyrrole [60] have been used. Morphology of(bio)degradable nanofibrous layers mainly by SEM or TEMhas been investigated. *e quality of nanofibres in the senseof homogeneity/heterogeneity, the beads presence, directingof the nanofibres, average diameter of nanofibres, distri-bution of active compounds in the nanofibres structures orcontinuity of coatings, presence of agglomerates, and theirsize has been evaluated. SEM investigations showed that theconductivity of final products was influenced by the con-tinuity of the coating [60, 91] and/or the distribution of theconductive substance [89, 90]. *e individual componentscould be distinguished within the fibres by TEM for theirdifferent electron density [88]. According the SEM images,the outgrowth and viability of cells in the samples afterelectrical stimulation were also assessed and thus the suit-ability of final product as a good candidate for electricalconductive scaffolds was determined [90].

5. Conclusions

*e (bio)degradable plastics market is relatively new, butinvolves more and more products that have so far beenobtained from nonbiodegradable conventional polymersand is divided mainly between PLA, starch blends, PCL,cellulose, poly(butylene succinate) (PBS), PHAs, and theirblends. Advanced (bio)degradable polymer materials areconsidered for many new applications, e.g., as supercon-ductors or engineering materials. Such materials are alsoregarded necessary in medical applications. *e develop-ment of polymeric materials for new applications is there-fore of great importance. However, efficient use requiresbasic research necessary to determine the relationship be-tween the structure of such materials, properties, andmechanism of degradation. Although environmentallyfriendly polymers have gained much attention as possiblesubstitutes for conventional plastics, factors that affect theirdurability are often difficult to investigate. Obtaining basicknowledge about these interactions will allow better tar-geting of innovative applications of advanced (bio)degrad-able polymeric materials and avoiding problems when usingfinal products. Various techniques are used to analysepolymeric materials. *e SEM and AFM techniques are anindispensable tool in studies of polymer blends, composites,and hybrid functional materials such as nanomaterials andcharacterisation of electrospun nanofibres.

For (bio)degradable polymers, microscopic techniquescan provide an accurate scan of surface morphology and givedetailed information on sample surfaces heterogeneity atnanometre resolution, providing information about mor-phological changes during, for example, processing ordetecting the elasticity and viscosities of a sample in the caseof different substrates or distinguishing polymer phasetransition. In addition, microscopic techniques such as AFMor SEM can be used to characterise blend, composite, andhybrid materials in terms of mechanical, physical, thermal,and chemical properties [56].

12 Advances in Materials Science and Engineering

Conflicts of Interest

*e authors declare that there are no conflicts of interestregarding the publication of this article.

Acknowledgments

*is work was supported by a Polish-Hungarian Joint Re-search Project “Controlled release and degradation studiesof biodegradable aliphatic polyester derivatives basednanoparticles loaded with organic drug” (2017–2019) and aPolish-Slovak Joint Research Project “Predictive study undercomposting conditions of bioactive materials obtained byelectrospinning” (2019–2021). *e authors would like tothank VEGA 2-0135-19 and APVV 15-0545 projects for thesupport.

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